As far as I can tell, swiftshader doesn't use LLVM's threading capabilities
for anything, and disabling them sidesteps an issue with libc++/win -- so
turn them off.

Bug: chromium:944811
Change-Id: Idf7eee5aa4693e9d2dcb744638ad9de53f714173
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27849
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nico Weber <thakis@chromium.org>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>